In the manufacture of pre-tensioned, pre-stressed concrete structural members it is conventional to provide wedge means external to the piece to be cast for holding the tensioned steel from movement after the tensioning apparatus is removed from the strand, which may be a wire, cable, rod or the like and which is referred to hereinafter, as in the industry, by the term "the steel."
The release of tension on the steel causes two phenomena to occur, the first being that the embedded steel tends to expand in diameter, and the other being that the steel tends to move into the cast concrete in a direction longitudinal to the steel. These effects result in weakening of the concrete in a zone surrounding the strand and extending inwardly from the external surface of the cast member for a distance which may be a large multiple of the diameter of the steel. This weakened zone is known in the industry as the "development length."
In FIG. 1 of the drawings there is shown, for purpose of illustration of the prior art, the conventional assembly of parts used in the manufacture of pre-tensioned, pre-stressed pre-cast concrete structural members. Such an assembly includes a form 2 within which the concrete member is cast, and which is closed by end plates 4, each of which is provided with one or more openings 6 leading to the interior of the form. A tensioned steel strand 8 is passed through the openings 6 and through the form and protrudes from the plate at each end of the form. Outside each end of the form there is provided means for holding the tensioned steel, and each such means comprises a wedge assembly including barrel 10 which rests on a fixed buttress 11 and has an internal frusto-conical surface 12 on which a plurality of wedges 14 are positioned in surrounding relation to the steel 8. A tensioning force in the direction of the arrow of FIG. 1 is applied to the steel, the concrete is cast within the form, and after the concrete is fully set the tensioning force is released and the wedge assemblies are removed. The purpose of the wedge assemblies to prevent retraction of the steel into the cast body of concrete, but it has been found that when the tensioning force is released the steel tends to enlarge in diameter and to retract into the cast body, and these effects cause a weakening of the cast body in a zone extending from the inner surface of each end plate inwardly of the cast body for a distance that may be as much as 100 times the diameter of the steel. This zone is known in the industry as the "development length" and is so designated in FIG. 1.
It is usually desirable or necessary to attach an end plate to the cast structural member, and because of the existence of the development length zone additional means must be provided for securing such an end plate to the structural member. This is conventionally done by welding one or more elongated rods 20 to the inner surface of the end plate, each rod having a length greater than the depth of the development length zone, as shown in FIG. 1. The attachement of this reinforcing and connecting rod may be effected by welding or other suitable means before the form is closed and the casting begins.
It has been the object of this invention to provide apparatus by which the development length zone is prevented in the manufacture of concrete structural members such as beams, columns, piles and the like. The elimination of the development length zone has important advantages which may not be immediately apparent. For example, it is often desirable and necessary to connect two such structural members in end-to-end relation, and in order to do this it is necessary to secure end plates to the structural members. In following known procedures in the manufacture of such pre-stressed members the formation of the development length zone at the end of the member prevents secure attachment of an end plate to the member without the use of additional attachment means, while utilization of the present invention results in attachment of each end plate to the cast body by the tensioned steel itself without the necessity of any additional connecting means. Further, the formation of the development length zone following conventional procedures and using known apparatus results in the weakening of the end parts of the structural member, thereby requiring special techniques in building and other structures, such as the use of special reinforcing rods within the development length zone. This is entirely obviated by the use of the present invention, as with such use the zone is eliminated.